The hoop winding technique consists in winding a reinforcing element, generally in form of a polymer-coated fibre tape, around a metal core so as to increase the internal pressure resistance of the core without significantly increasing the weight thereof, considering the low weight of the tapes.
The core can be a metal tube, made of steel for example. The reinforcing element is an elongate element. It can come in form of a strip, a wire, or preferably a wire assembly or a yarn coated with a polymer matrix.
According to an embodiment of the hoop winding technique, the reinforcing element is wound around the core while applying a tension. Thus, the reinforcing element wound around the core undergoes a tensile stress, which causes the metal core to undergo a compressive stress. The prestress undergone by the core is similar to the prestress that would be produced by an outside pressure.
Subsea reservoir drilling operations are carried out using a riser pipe allowing the blowout preventer located on the seabed to be connected to the sea surface. The riser is fitted with at least two auxiliary lines referred to as kill line (KL) and choke line (CL), whose main purpose is to provide a hydraulic connection between the sea surface and the sea bottom. More particularly, auxiliary lines make it possible to supply the well with fluid by circulating below a closed blowout preventer and/or to discharge a fluid from the well, without flowing through the riser pipe that does not withstand high pressures. The fluid thus carried, resulting from a kick from an underground reservoir, can circulate at a pressure that can be above 700 bars.
It has been suggested to use hoop-wound tubes for the auxiliary lines of a drilling riser, notably in patents FR-2,828,262 and FR-2,828,121 filed by the applicant. Manufacturing a high-pressure line element from a hoop-wound tube is both simple and economical. FIG. 1 shows an example embodiment disclosed in the prior art. The line element comprises four distinct parts: a metal tube 1 or core, a first connecting means 3, a second connecting means 4 and hoop layers 2. Connecting means 3 is a female connecting means suited to be connected to a male pin 12 fastened to connecting means 4. Tube 1 is manufactured from a rolled blank for example, then connecting means 3 and 4 are directly welded onto ends 8 and 9 of tube 1. Hoop winding of the metal assembly is then achieved by winding a reinforcing element around tube 1 and around part of connecting means 3 and 4. Only the outer parts 10 and 11 of the connecting means are not hooped and they have sufficient thickness to withstand at least the same internal pressure as the hoop-wound part.
Thus, making such a pipe requires connecting means 3 and 4 whose ends to be welded have dimensions (diameters and thickness) that are suited to tube 1, so that they can be welded thereto directly. These connecting means 3 and 4 have to be mechanically compatible with the stress imposed by hoop winding on transition zones 5 and 7. However, the shapes and the dimensions of connecting means 3 and 4 are also imposed by their operational specifications and by the specific links intended for integration of the safety lines on the riser joint, in particular cooperation with the riser joints. All the constraints relative to the connecting means require a specific and complex implementation of these connecting means.
It is therefore not possible to standardize the design of high-pressure line elements according to the prior art (KL and CL) that require connections specific to the riser.
Another major constraint relative to pipes is their high pressure resistance. Typically, the steel pipes currently used for riser auxiliary lines are designed to withstand pressures of the order of 15,000 psi (i.e. 103 MPa). However, some new applications require auxiliary lines designed to withstand pressures of the order of 20,000 psi (i.e. 138 MPa). Current connecting means, notably those described in patent applications FR-2,828,262 and FR-2,828,121 filed by the applicant, do not allow to withstand such pressures.
Furthermore, adapting these configurations for higher pressures would require increasing the connecting means thickness. This would result in an increase in size and weight if the thickness is increased by increasing the outside diameter, and in a decrease in diameter and therefore a pressure drop increase if the thickness is increased by decreasing the inside diameter. The latter option would lead to a significant passage diameter reduction for the choke and kill lines, at the end fitting, of at least 1″, i.e. 2.54 cm (3.5″, i.e. 8.89 cm instead of 4.5″, i.e. 11.43 cm). This reduction would cause pressure drops that might complicate blowout control operations. To a first approximation, it can be estimated that the pressure drops on turbulent flow are proportional to the length of line L, to the square of flow rate Q and inversely proportional to the diameter to the 5th power.
      Δ    ⁢                  ⁢    P    =      k    ⁢                  LQ        2                    D        5            
A local reduction of the inside diameter by 3.5″ instead of 4.5″ over a length of 3 ft, i.e. 91.4 cm, increases the pressure drops by about 10%. This increase reaches 26% for a 3″, i.e. 7.62 cm, diameter and only 3% for a 4″, i.e. 10.16 cm, diameter.
Furthermore, there is a need to adapt an existing riser so that it withstands higher pressures.
The present invention relates to a line element comprising a hooped tube provided at both ends thereof with two receptacles in which male and female end fittings made of high-strength steel are fastened. Using the hoop winding technique allows to limit the weight of the line while improving the pressure resistance of the line element. Equipping the connection with high-strength end fittings also provides pressure resistance of the line element, while limiting the thickness and therefore the weight of the connecting means.
The invention also relates to a riser section comprising at least one auxiliary line element having such characteristics. Furthermore, the invention relates to a method for upgrading a riser pipe by replacing an auxiliary line element with a line element having such characteristics.